An army of undercover mothers is to be recruited to go into nurseries and report back on the food after concerns that some youngsters are being fed on a budget of just 25p per meal.

Mothers posing as prospective parents will check up on the quality of the meals and snacks being served, as part of the Better Nursery Food Now campaign, after concerns that children in nurseries were eating junk food which would be banned in schools.

Campaigners have launched a Facebook campaign to call on the Government to set clear nutritional standards for all nurseries, and for parents to post their experiences of their children's food at nursery. Its petition has already attracted 4,338 signatures. The campaign, run by the Soil Association and the food company Organix, is demanding that the Government set mandatory standards for nursery meals.

A Soil Association spokeswoman said: "There are all these amazing things that have happened for school meals, but nothing for children in nurseries – who are arguably the most vulnerable. We think this is unacceptable, and we want changes to be made urgently."

Organix, which makes organic baby food, has already asked its online community of about 4,000 mothers to undertake undercover inspections of nursery food, and hopes many more will join the campaign. Anna Guyer, a spokeswoman for Organix, said: "We are looking for undercover mums who could go around two or three nurseries and complete a questionnaire about the food. A lot of people who are really interested in organic food at home say that it wasn't something that they even thought about when they chose their child's nursery, and yet it is so very important."

The campaign's 2008 report, Georgie Porgy Pudding and Pie, was the first ever study that looked in detail at the quality of food given to children attending nurseries in the UK. It found that some nurseries spent as little as 25p per meal per child. It also discovered that colourings and additives that are banned in manufactured foods for children were regularly being permitted in nurseries. Foods such as chips, sweets and chocolate were still regularly served in nurseries. A spokeswoman said: "Since the publication of the report, despite widespread support from parents, the Government has failed to take the lead on the crucial issue of nourishing the next generation."

There is currently no compulsory training for nursery staff serving food, no clear nutritional standards and no agency to monitor the quality of food provided. There are approximately 13,600 nurseries in England, with more than 588,000 children attending. On average parents pay £159 per week, or just over £31 a day, for a child under two to go to nursery. The report found that a very small fraction of this cost goes on food: on average just three to six per cent of the cost of sending a child to nursery is spent on food for the child.

Gemma Spratt, a mother who has posted her experiences on the campaign's Facebook page, wrote: "My daughter doesn't go to a nursery and this was one of the reasons why. I was told she would be given cakes and I would have to manage her behaviour if she kicked off because I wouldn't allow her to have cake. This is even more key than school dinners in my eyes as if we can start them off with good eating habits then that's a lifelong love of healthy food."

A spokesman for the Department for Children, Schools and Families said there was already a legal requirement for all childcare providers to offer healthy and nutritious meals. He added: "Ofsted inspects all childcare settings on these legal requirements, and they would take action if they were not being met. We trust their judgement and are encouraged by their own survey, which rated the majority of providers as good or outstanding in relation to providing a healthy diet. They concluded that child carers have embraced the concept of healthy eating and said that the picture looks good for younger children."